enhancement: add metrics via a configuration file

New option to add custom metrics from TOML file. Add examples and update
README file. You can also use env var.

Refactor the way we retrieve informations from oracle by introducing new
functions ScrapeGenericValues and GeneratePrometheusMetrics to simplify
Prometheus metrics generation.

Porting several existing function to use ScrapeGenericValues and removing
specific methods to scrape values.

Introducing new Metric struct describing metrics to scrape.
Add a new structure with all metrics defined.

Stop using a new connection everytime we scrape values. Go handle case where
connections is lost.

New metrics to retrieve asm diskgroup informations and resource limit.

Update version of ubuntu in Dockerfile.

Author: Yannig Perré

Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:17.04
+FROM ubuntu:17.10
 MAINTAINER Seth Miller <seth@sethmiller.me>
 
 RUN apt-get -qq update && \

Makefile

@@ -20,6 +20,7 @@ deps:
 	@sudo rpm -Uvh --nodeps oracle*rpm
 	@echo /usr/lib/oracle/12.2/client64/lib | sudo tee /etc/ld.so.conf.d/oracle.conf
 	@sudo ldconfig
+	@PKG_CONFIG_PATH=${PWD} go get
 
 test:
 	@echo test

README.md

@@ -31,6 +31,8 @@ The following metrics are exposed currently.
 - oracledb_tablespace_bytes
 - oracledb_tablespace_max_bytes
 - oracledb_tablespace_bytes_free
+- oracledb_resource_current_utilization
+- oracledb_resource_limit_value
 
 # Installation
 
@@ -67,12 +69,99 @@ Usage of oracledb_exporter:
        	If set use a syslog logger or JSON logging. Example: logger:syslog?appname=bob&local=7 or logger:stdout?json=true. Defaults to stderr.
   -log.level value
        	Only log messages with the given severity or above. Valid levels: [debug, info, warn, error, fatal].
+  -custom.metrics string
+        File that may contain various custom metrics in a TOML file.
   -web.listen-address string
        	Address to listen on for web interface and telemetry. (default ":9161")
   -web.telemetry-path string
        	Path under which to expose metrics. (default "/metrics")
 ```
 
+# Custom metrics
+
+This exporter does not have the metrics you want? You can provide new one using TOML file. To specify this file to the exporter, you can:
+- Use ``-custom.metrics`` flag followed by the TOML file
+- Export CUSTOM_METRICS variable environment (``export CUSTOM_METRICS=my-custom-metrics.toml``)
+
+This file must contain the following elements:
+- One or several metric section (``[[metric]]``)
+- For each section a context, a request and a map between a field of your request and a comment.
+
+Here's a simple example:
+
+```
+[[metric]]
+context = "test"
+request = "SELECT 1 as value_1, 2 as value_2 FROM DUAL"
+metricsdesc = { value_1 = "Simple example returning always 1.", value_2 = "Same but returning always 2." }
+```
+
+This file produce the following entries in the exporter:
+
+```
+# HELP oracledb_test_value_1 Simple example returning always.
+# TYPE oracledb_test_value_1 gauge
+oracledb_test_value_1 1
+# HELP oracledb_test_value_2 Same but returning always 2.
+# TYPE oracledb_test_value_2 gauge
+oracledb_test_value_2 2
+```
+
+You can also provide labels using labels field. Here's an example providing two metrics, with and without labels:
+
+```
+[[metric]]
+context = "context_no_label"
+request = "SELECT 1 as value_1, 2 as value_2 FROM DUAL"
+metricsdesc = { value_1 = "Simple example returning always 1.", value_2 = "Same but returning always 2." }
+
+[[metric]]
+context = "context_with_labels"
+labels = [ "label_1", "label_2" ]
+request = "SELECT 1 as value_1, 2 as value_2, 'First label' as label_1, 'Second label' as label_2 FROM DUAL"
+metricsdesc = { value_1 = "Simple example returning always 1.", value_2 = "Same but returning always 2." }
+```
+
+This TOML file produce the following result:
+
+```
+# HELP oracledb_context_no_label_value_1 Simple example returning always 1.
+# TYPE oracledb_context_no_label_value_1 gauge
+oracledb_context_no_label_value_1 1
+# HELP oracledb_context_no_label_value_2 Same but returning always 2.
+# TYPE oracledb_context_no_label_value_2 gauge
+oracledb_context_no_label_value_2 2
+# HELP oracledb_context_with_labels_value_1 Simple example returning always 1.
+# TYPE oracledb_context_with_labels_value_1 gauge
+oracledb_context_with_labels_value_1{label_1="First label",label_2="Second label"} 1
+# HELP oracledb_context_with_labels_value_2 Same but returning always 2.
+# TYPE oracledb_context_with_labels_value_2 gauge
+oracledb_context_with_labels_value_2{label_1="First label",label_2="Second label"} 2
+```
+
+Last, you can set metric type using **metricstype** field.
+
+```
+[[metric]]
+context = "context_with_labels"
+labels = [ "label_1", "label_2" ]
+request = "SELECT 1 as value_1, 2 as value_2, 'First label' as label_1, 'Second label' as label_2 FROM DUAL"
+metricsdesc = { value_1 = "Simple example returning always 1 as counter.", value_2 = "Same but returning always 2 as gauge." }
+# Can be counter or gauge (default)
+metricstype = { value_1 = "counter" }
+```
+
+This TOML file will produce the following result:
+
+```
+# HELP oracledb_test_value_1 Simple test example returning always 1 as counter.
+# TYPE oracledb_test_value_1 counter
+oracledb_test_value_1 1
+# HELP oracledb_test_value_2 Same test but returning always 2 as gauge.
+# TYPE oracledb_test_value_2 gauge
+oracledb_test_value_2 2
+```
+
 # Integration with Grafana
 
 An example Grafana dashboard is available [here](https://grafana.com/dashboards/3333).